Understanding how the AC control lever works

[davepl]

Could someone tell me what happens on a properly set up truck when the control lever is moved from "Inside" to "Outside"? My expectation is that "Inside" recirculates air, so it closes the kick panel and cowl doors for "Inside" and opens them for "Outside".

My problem is that in 1970 you only have a single vacuum port on the control head (2 ports, one in, one out). You can apply vacuum to all three or none of them. So if you close the doors by removing the vacuum signal, you also open the heater valve.

So you can have the doors open and the heater valve closed (cool fresh outside air) or the doors closed and the valve open (recirclated heated air).

None of that gives you the "Inside" mode of recirculated cold air. I'm at a loss how it's even possible, since the water valve closes with vacuum present.

In summary:

UNDER VACUUM:
- Heater valve closes
- Kick panel and cowl doors open

NO VACUUM:
- Heater valve reopens
- Kick panel and cowl doors close

How do you make an "Inside" mode of cool recirculated air out of that? I actually have the 3-port control head, and only using 2 of them, but the truck didn't originally have the 3-port (which is intended for the water valve that opens on vacuum), and it worked from the factory!

Let's stick solely with the simpler 2-port head for now; that's intended for use with the water valve that closes under vacuum (unless I'm wrong on that key point).

My only flailing guess is that the heater valve is actually OPEN during Recirc AC, but the airflow takes a different path, so it doesn't matter. But I have no idea.